A metered amount of foam concentrate can be injected into a conduit such as a pipe or hose through which water to be used in fighting a fire is flowing so that a foam can result from the water and concentrate mixture that will much more efficiently extinguish the fire than water alone. It is desirable that such concentrate only be injected into the water when the water is flowing above a predetermined rate through the conduit, that the concentrate is injected within seconds after water flow through the conduit reaches that predetermined rate which affords minimizing the amount of water that will be directed at certain types of fires, and that the injection be terminated within seconds after flow through the conduit drops below that predetermined rate which minimizes the amount of foam concentrate used and insures that the concentrate will not flow in the wrong direction along the conduit to potentially contaminate an adjacent pumping system for the water.
Manual activation and deactivation of the injection system is impractical as it presents too many opportunities for operator error. Thus it is desirable to use an automatic activation and deactivation system including a flow rate detector that activates or deactivates the injection system at the appropriate times. Impeller or paddle switches are not suitable as the flow rate detector in such systems, however, because solid particles such as sand and small rocks carried by the water flowing through the conduit can damage or wear down the portions of the impeller or paddle switch exposed to the water flow. While ultrasonic doppler sensors or deferential pressure sensors could possibly be used as the flow rate detectors in such systems, neither of these sensors would be practical or cost effective for the application described above.